The label by the apparatus reads "Calorimeter by James Joule/1876/Used in his final determination". In the background are reproductions of illustrations from his 1876 determination of the mechanical equivalent of heat.
James Prescott Joule (1818-1889) had used a number of techniques to measure this quantity. His best known method used rotating paddles to stir water in a calorimeter. Thermometers of his own devising were used to measure the resulting temperature increase of the water, and falling weights provided the necessary mechanical energy. He first used this method in preliminary experiments in 1847 and in what he originally though were definitive measurements published in 1849. In the 1870s he returned to the technique, and used this calorimeter for his final determination of the mechanical equivalent of heat, in which he obtained a value of 4.1538 Joules/Kelvin. Of course, he did not use these units!
